At a dramatic press conference in the Sheraton hotel in Boston, just a couple of blocks away from the bomb site, the FBI's Boston chief, Richard DesLauriers, unveiled photographs of two young men whom he called "Suspect 1" and "Suspect 2". By studying footage from the crime scene, he said, the FBI had concluded that the two were associated with each other, and that Suspect 2 put down a backpack at the site of the second explosion, just in front of the Forum restaurant, shortly before the blast occurred.

Following apparently planting the bomb, Suspect 2 walked west along Boylston Street and then disappeared from police sight. Video released by the FBI on its website showed Suspect 1 walking through the marathon crowd, followed by Suspect 2 about five paces behind. The two men do not talk to each other but do move in the same direction, both with bulky packs slung on their shoulders.

In the photographs, the two suspects are seen walking through the crowd of marathon spectators up Boylston Street, in the vicinity of Gloucester Street. Each suspect is the subject of four photographs: Suspect 1 is heavy set, wearing sunglasses and a black baseball cap with a white insignia in front; he is dressed in a white T-shirt and a knee-length dark coat and khaki-coloured trousers. Suspect 2 is thinner, with dark curly hair protruding from a white baseball cap placed backwards on his head. He is dressed in a grey hoody beneath a black coat and black trousers.

DesLauriers stressed that the two men were armed and extremely dangerous, and warned the public not to approach them in any circumstances. But he also repeated his appeal for somebody to come forward.

"For more than 100 years the FBI has relied on the public to be its eyes and ears," he said. "The public will play a critical role. Somebody out there knows these individuals as family, friends or neighbours. It may be difficult, but the nation is relying on the public to come forward."

The release of the suspect photos propels the hunt for the Boston marathon bombers into a new phase. DesLauriers said there would be no more media briefings for the foreseeable future, as the 1,000 FBI agents assigned to the chase would be focusing exclusively on apprehending the men. "Identifying and locating those responsible is now our highest responsibility. It continues to be an ongoing active investigation," he said.

DesLauriers stressed that he was aware of no further threats to the public. But the country remains jittery, with a spate of recent evacuations of public buildings including the federal courthouse in Boston on Wednesday.

DesLauriers said the FBI had initially focused on a single suspect, having pored through thousands of hours of video and followed thousands of tips. But agents had eventually been led to a second suspect, who appeared to be associated with the first.